Process for producing benzene from a C5-C12 hydrocarbon mixture

ABSTRACT

The invention relates to a process for producing benzene, comprising the steps of:
     (a) providing a hydrocracking feed stream comprising C5-C12 hydrocarbons,   (b) contacting the hydrocracking feed stream in the presence of hydrogen with a hydrocracking catalyst comprising 0.01-1 wt-% hydrogenation metal in relation to the total catalyst weight and a zeolite having a pore size of 5-8 Å and a silica (SiO 2 ) to alumina (Al 2 O 3 ) molar ratio of 5-200 under process conditions including a temperature of 425-580° C., a pressure of 300-5000 kPa gauge and a Weight Hourly Space Velocity of 0.1-15 h −1  to produce a hydrocracking product stream comprising benzene, toluene and C8+ hydrocarbons,   (c) separating benzene, toluene and the C8+ hydrocarbons from the hydrocracking product stream and   (d) selectively recycling back at least part of the toluene from the separated products of step (c) to be included in the hydrocracking feed stream.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of application Ser. No.15/318,136, filed Dec. 12, 2016, which is a 371 of InternationalApplication No. PCT/EP2015/062080, filed Jun. 1, 2015, which claimspriority to European Application No. 14172342.9, filed Jun. 13, 2014both of which are incorporated herein by reference in their entirety.

The present invention relates to a process for producing benzene from amixed feedstream comprising C5-C12 hydrocarbons by contacting saidfeedstream in the presence of hydrogen with a catalyst havinghydrocracking activity.

WO2013/182534 discloses a process for producing BTX (benzene, tolueneand xylene) from a C5-C12 hydrocarbon mixture using ahydrocracking/hydrodesulphurisation catalyst. According toWO2013/182534, the process results in a mixture comprising substantiallyno co-boilers of BTX, thus chemical grade BTX can easily be obtained.

While the process of WO2013/182534 advantageously limits the amount ofBTX co-boilers in the obtained mixture, control of the ratio betweenbenzene, toluene and xylene in the obtained mixture is limited. In somecases, it is preferred to obtain more benzene in the product than waspresent in the feed and/or to eliminate any benzene losses through theprocess. WO2013/182534 also mentions separating the BTX from the mixtureand contacting the separated BTX with hydrogen under conditions suitableto produce a hydrodealkylation product stream comprising benzene andfuel gas. This results in a high benzene yield, but requires tworeactors in series, as hydrocracking and hydrodealkylation is performedunder different reaction conditions The use of more reactors typicallyleads to a higher capital expenditure (CAPEX).

CN101734986 discloses a method for pyrolysing C7+ hydrocarbons in thepresence of a catalyst for obtaining a product stream rich in benzeneand xylene. CN101734986 mentions that the obtained product streamcomprises a large quantity of toluene, as well as heavy hydrocarbons(C9+) and non-aromatic hydrocarbons which are of less value. Accordingto CN101734986, the heavy hydrocarbons (C9+) are recycled back into thereaction zone. In some cases, toluene is also recycled back. Therecycling of these materials improves the benzene and xylene yield.

US2006/0287564 describes a process for increasing the production ofbenzene from a hydrocarbon mixture including separating a hydrocarbonfeedstock into a C6 or lower hydrocarbon stream and a C7 or higherhydrocarbon stream. The C6 or lower hydrocarbon stream is separated intoa non-aromatic hydrocarbon stream and an aromatic hydrocarbon streamthrough a solvent extraction process. The C7 or higher hydrocarbonstream is subjected to a reaction in the presence of a catalystcomprising platinum/tin or platinum/lead.

U.S. Pat. No. 3,957,621 describes a process for processing heavyreformates from which benzene and lighter components have been largelyremoved. The removed stream (overhead stream at line 12 of FIG. 1)includes the major portion of the benzene in the charge and can includea substantial portion of the toluene (col. 4, l. 65-69).

It would be desirable to provide a process for converting a C5-C12hydrocarbon feed stream by which chemical grade benzene is obtained,which results in an increased yield of benzene compared to knownprocesses. It would also be desirable to provide a process whichminimizes the capital expenditure required.

It is an object of the present invention to provide a process forconverting a C5-C12 hydrocarbon feed stream into a product streamcomprising benzene in which the above and/or other needs are met.

Accordingly, the present invention provides a process for producingbenzene comprising the steps of:

(a) providing a hydrocracking feed stream comprising C5-C12hydrocarbons,

(b) contacting the hydrocracking feed stream in the presence of hydrogenwith a hydrocracking catalyst comprising 0.01-1 wt-% hydrogenation metalin relation to the total catalyst weight and a zeolite having a poresize of 5-8 Å and a silica (SiO₂) to alumina (Al₂O₃) molar ratio of5-200 under process conditions including a temperature of 425-580° C., apressure of 300-5000 kPa gauge and a Weight Hourly Space Velocity of0.1-15 h⁻¹ to produce a hydrocracking product stream comprising benzene,toluene and C8+ hydrocarbons,(c) separating benzene, toluene and the C8+ hydrocarbons from thehydrocracking product stream and(d) selectively recycling back at least part of the toluene from theseparated products of step (c) to be included in the hydrocracking feedstream.

The term ‘selectively recycle’ is herein understood to mean that onlytoluene is recycled back to be included in the hydrocracking feed streamamong the separated products of step (c) comprising benzene, toluene andC8+ hydrocarbons.

Depending on the hydrocracking reaction conditions and the ratio ofbenzene/toluene/xylene in the hydrocracking feed stream, the conversionof toluene into benzene and xylene (toluene disproportionation) may alsooccur. In case toluene content of the feed stream is limited, the extentof toluene disproportionation will be limited and hydrocracking will beassociated with a benzene loss. By recycling back the toluene separatedfrom the hydrocracking product stream into the hydrocracking feedstream, the benzene to toluene ratio of the hydrocracking feed streamcan be manipulated and the extent of the toluene disproportionation canbe increased. This leads to an increase in the benzene yield.

Unlike the use of a hydrodealkylation reactor after hydrocracking toconvert toluene and xylene to benzene as in WO2013/182534, the benzeneyield is increased according to the process of the invention byrecycling back of toluene using only hydrocracking reactor. This isbeneficial for the reduction of CAPEX. Additionally, toluenedisproportionation occurring during the process according to theinvention produces significantly lower levels of fuel gas compared tohydrodealkylation.

C8+ hydrocarbons in the hydrocracking product stream comprise xylene.The mild hydrocracking conditions as used according to step b) of theinvention result in a product stream comprising benzene, toluene andxylene (BTX), other C8+ hydrocarbons and a low amount of BTX co-boilers.Because of the absence of BTX co-boilers the BTX is a ‘chemical gradeBTX’. The benzene obtained is a chemical grade benzene. Similarly, thetoluene obtained is a chemical grade toluene.

According to the process of the invention, only the toluene is recycledback to be included in the hydrocracking feed stream. This has anadvantageous effect on the life time of the hydrocracking catalyst.Heavier hydrocarbons such as C9+ hydrocarbons quickly reduce the lifetime of the hydrocracking catalyst used in the process of the invention.The hydrocracking feed stream used according to the process of theinvention is a lighter feed stream than the feed stream used in theprocess of CN101734986 that comprises C7+ hydrocarbons. The initial lowamount of C9+ components in the feed stream also has an advantageouseffect on the life time of the hydrocracking catalyst.

Xylene and the C9+ components in the feed stream will not havebeneficial impact on the benzene yield, and can even decrease thebenzene yield through e.g. the transalkylation reaction oftrimethylbenzene and xylene. Hence, selectively recycling back tolueneis advantageous for a higher benzene yield rather than recycling backtoluene together with xylene and C9+ components.

As used herein, the term “C# hydrocarbons”, wherein “#” is a positiveinteger, is meant to describe all hydrocarbons having # carbon atoms.Moreover, the term “C#+ hydrocarbons” is meant to describe allhydrocarbon molecules having # or more carbon atoms. Accordingly, theterm “C5+ hydrocarbons” is meant to describe a mixture of hydrocarbonshaving 5 or more carbon atoms.

The term “BTX” as used herein is well known in the art and relates to amixture of benzene, toluene and xylenes.

As used herein, the term “chemical grade BTX” relates to a hydrocarbonmixture comprising less than 5 wt-% hydrocarbons other than benzene,toluene and xylenes, preferably less than 4 wt-% hydrocarbons other thanbenzene, toluene and xylenes, more preferably less than 3 wt-%hydrocarbons other than benzene, toluene and xylenes, and mostpreferably less than 2.5 wt-% hydrocarbons other than benzene, tolueneand xylenes.

Furthermore, the “chemical grade BTX” produced by the process of thepresent invention comprises less than 1 wt-% non-aromatic C6+hydrocarbons, preferably less than 0.7 wt-% non-aromatic C6+hydrocarbons, more preferably less than 0.6 wt-% non-aromatic C6+hydrocarbons and most preferably less than 0.5 wt-% non-aromatic C6+hydrocarbons. The most critical contaminants are the non-aromaticspecies which have boiling points close to benzene including, but notlimited to, cyclohexane, methylcyclopentane, n-hexane, 2-methylpentaneand 3-methylpentane.

Accordingly, the hydrocracking product stream is substantially free fromnon-aromatic C6+ hydrocarbons. As meant herein, the term “product streamsubstantially free from non-aromatic C6+ hydrocarbons” means that saidproduct stream comprises less than 1 wt-% non-aromatic C6+ hydrocarbons,preferably less than 0.7 wt-% non-aromatic C6+ hydrocarbons, morepreferably less than 0.6 wt-% non-aromatic C6+ hydrocarbons and mostpreferably less than 0.5 wt-% non-aromatic C6+ hydrocarbons.

According to the present invention, chemical grade benzene can also beeasily separated from the hydrocracking product stream. As used herein,the term “chemical grade benzene” relates to a hydrocarbon streamcomprising less than 0.5 wt % hydrocarbons other than benzene.

According to the present invention, chemical grade toluene can also beeasily separated from the hydrocracking product stream. As used herein,the term “chemical grade toluene” relates to a hydrocarbon streamcomprising less than 0.5 wt % hydrocarbons other than toluene.

The term “aromatic hydrocarbon” is very well known in the art.Accordingly, the term “aromatic hydrocarbon” relates to cyclicallyconjugated hydrocarbon with a stability (due to delocalization) that issignificantly greater than that of a hypothetical localized structure(e.g. Kekulé structure). The most common method for determiningaromaticity of a given hydrocarbon is the observation of diatropicity inthe 1H NMR spectrum, for example the presence of chemical shifts in therange of from 7.2 to 7.3 ppm for benzene ring protons.

The hydrocracking product stream produced in the process of the presentinvention preferably comprises less than 10 wt-% of methane. Morepreferably, the first product stream produced in the process of thepresent invention comprises less than 5 wt-% of methane, more preferablyless than 4 wt-% of methane, more preferably less than 3 wt-% methane,even more preferably less than 2 wt-% methane, particularly preferablyless than 1.5 wt-% methane and most preferably less than 1 wt-% methane.

Preferably, the hydrocracking product stream is also substantially freefrom C5 hydrocarbons. As meant herein, the term “product streamsubstantially free from C5 hydrocarbons” means that said hydrocrackingproduct stream comprises less than 1 wt-% C5 hydrocarbons, preferablyless than 0.7 wt-% C5 hydrocarbons, more preferably less than 0.6 wt-%C5 hydrocarbons and most preferably less than 0.5 wt-% C5 hydrocarbons.

Step a)

According to step a) of the process according to the invention ahydrocracking feed stream comprising C5-C12 hydrocarbons is provided.

Hydrocracking Feed Stream

The hydrocracking feed stream used in the process of the presentinvention is a mixture comprising C5-C12 hydrocarbons, preferably havinga boiling point in the range of 30-195° C. The hydrocracking feed streamcomprises the toluene recycled back from the hydrocracking productstream. Preferably, the hydrocracking feed stream mainly comprises C6-C8hydrocarbons.

Step (a) involves mixing a fresh feed stream and the recycled toluene.The fresh feed stream may comprise at least 10 wt % of benzene, at least20 wt % of benzene or at least 30 wt % of benzene. Suitable fresh feedstreams include, but are not limited to first stage or multi-stagehydro-treated pyrolysis gasoline, straight run naphtha, hydrocrackedgasoline, light coker naphtha and coke oven light oil, FCC gasoline,reformate or mixtures thereof.

It is preferred that the non-aromatic species comprised in thehydrocracking feed stream are saturated (e.g. by the priorhydrogenation) in order to reduce the exotherm within the catalyst bedcontaining the hydrocracking catalyst used in the present process.Accordingly, the hydrocracking feed stream is preferably provided byhydrogenating a source feed stream in a hydrogenation reactor and mixingthe hydrogenated source feed stream (fresh feed stream) and the recycledtoluene. Suitable source feed streams include, but are not limited tofirst stage or multi-stage hydro-treated pyrolysis gasoline, straightrun naphtha, hydrocracked gasoline, light coker naphtha and coke ovenlight oil, FCC gasoline, reformate or mixtures thereof. The source feedstream may have a relatively high sulphur content, such as pyrolysisgasoline (pygas), straight run naphtha, light coker naphtha and cokeoven light oil and mixtures thereof.

Accordingly, preferably, the fresh feed stream is pyrolysis gasoline,straight run naphtha, light coker naphtha and coke oven light oil ormixtures thereof, optionally after being hydrogenated in a hydrogenationreactor. Preferably, the fresh feed stream has not been subjected to thestep of removing benzene.

Preferably, the hydrocracking feed stream has not been subjected to thestep of removing benzene. Preferably, the hydrocracking feed streamcomprises at least 1 wt % of benzene, for example at least 3 wt %, 5 wt%, 10 wt % or 15 wt %, and/or at most 35 wt %, at most 30 wt %, at most25 wt % or at most 20 wt %.

Hydrogenation of the source feed stream can be achieved by ahydrogenation reactor in series with the hydrocracking reactor or by areactor comprising a hydrogenation bed and a hydrocracking bed in seriesor a layer of a hydrogenation catalyst on top of the hydrocrackingcatalyst. The single reactor construction would imply lower capitalcosts compared to two reactors in series.

For instance, a typical composition of first stage hydro-treatedpyrolysis gasoline may comprise 10-15 wt-% C5 olefins, 2-4 wt-% C5paraffins and cycloparaffins, 3-6 wt-% C6 olefins, 1-3 wt-% C6 paraffinsand naphthenes, 25-30 wt-% benzene, 15-20 wt-% toluene, 2-5 wt-%ethylbenzene, 3-6 wt-% xylenes, 1-3 wt-% trimethylbenzenes, 4-8 wt-%dicyclopentadiene, and 10-15 wt-% C9+ aromatics, alkylstyrenes andindenes; see e.g. Table E3.1 from Applied Heterogeneous Catalysis:Design, Manufacture, and Use of Solid Catalysts (1987) J. F. Le Page.However, also hydrocarbon mixtures that are depentanised so theconcentrations of all the C6 to C9 hydrocarbon species are relativelyhigh compared with the typical FIGURES above can be advantageously usedas a feed stream in the process of the present invention.

In one embodiment, the hydrocracking feed stream used in the process ofthe present invention is treated so that it is enriched in mono-aromaticcompounds. As used herein, the term “mono-aromatic compound” relates toa hydrocarbon compound having only one aromatic ring. Means and methodssuitable to enrich the content of mono-aromatic compounds in a mixedhydrocarbon stream are well known in the art such as the Maxene process;see Bhirud (2002) Proceedings of the DGMK-conference 115-122.

The source feed stream or the hydrocracking feed stream used in theprocess of the present invention may comprise up to 300 wppm of sulphur(i.e. the weight of sulphur atoms, present in any compound, in relationto the total weight of the feed).

Step b)

According to step b) of the process according to the invention thehydrocracking feed stream is contacted in the presence of hydrogen in ahydrocracking reactor with a hydrocracking catalyst.

The hydrocracking feed stream is contacted with a hydrocracking catalystin the presence of hydrogen to produce a hydrocracking product stream.During hydrocracking of hydrocarbons, toluene disproportionationinfluences the proportion of benzene in the product stream:

The forward toluene disproportionation reaction increases the amount ofbenzene. This reaction occurs more in a stream having a low ratio ofbenzene to toluene.

Recycling back the toluene in step d) of the process of the invention tothe hydrocracking feed stream increases the amount of toluene in thehydrocracking feed stream. This results in a higher overall benzeneyield compared to the process wherein no recycling back of toluene takesplace.

By varying the amount of toluene to be recycled back, the amount oftoluene in the hydrocracking feed stream can be controlled. Bycontrolling the amount of toluene in the hydrocracking feed stream, itis advantageously possible to control the amount of benzene in thehydrocracking product stream.

In some embodiments, all of the toluene from the separated products ofstep (c) is recycled back to be included in the hydrocracking feedstream. In other embodiments, part of the toluene from the separatedproducts of step (c) is recycled back to be included in thehydrocracking feed stream.

Recycling of a higher proportion of toluene increases the benzene yield,but the presence of too much toluene in the hydrocracking feed streammay result in insufficient catalyst activity towards hydrocracking ofthe benzene coboilers, and hence not to the production of chemical gradebenzene. If all of the toluene from the separated products of step (c)is recycled back to be included in the hydrocracking feed stream, theamount of toluene in the hydrocracking feed stream may increase to suchan extent that a very large reactor is required for containing enoughcatalyst to perform both toluene disproportionation as well as benzenepurification, i.e. hydrocracking. By limiting the amount of toluene tobe recycled, the essential function of benzene purification, i.e.hydrocracking, by the catalyst can be preserved.

Hence, the proportion of the toluene to be recycled back is preferablyselected such that there is no benzene loss or a nominal benzene gainduring the hydrocracking while sufficient hydrocracking of the benzenecoboilers takes place.

Accordingly, the amount of toluene to be included in the hydrocrackingfeed stream may be set so that there is no benzene loss or a nominalbenzene gain. Accordingly, in some embodiments of the process of theinvention, the amount of toluene to be recycled back to be included inthe hydrocracking feed stream is set so that the proportion of benzenein the hydrocracking product stream is 0-50 mol %, for example 0-25 mol%, for example 0-10 mol %, for example 0-5 wt % higher than theproportion of benzene in the hydrocracking feed stream. This can beachieved by controlling parameter [BX]/T² described below. Therelationship between the [BX]/T² and the resulting difference in theproportion of benzene in the hydrocracking feed stream and in thehydrocracking product stream can be experimentally determined by theskilled person. The skilled person can accordingly control [BX]/T² forachieving the desired change in the proportion of benzene in thehydrocracking feed stream and in the hydrocracking product stream.

The toluene in the hydrocracking product stream which is not recycledback to be included in the hydrocracking feed stream may advantageouslybe subjected to a separate and dedicated toluene disproportionationprocess reactor or used for other purposes. The use of the dedicatedtoluene disproportionation reactor further increases the benzene yield.Accordingly, in some embodiments of the present invention, part of thetoluene from the separated products of step (c) is contacted withhydrogen under conditions suitable for toluene disproportionation in areactor separate from the reactor for performing the hydrocracking step(b).

The amount of the toluene to be recycled back to be included in thehydrocracking feed stream may e.g. be 10-80 wt %, for example 50-70 wt %of the toluene in the hydrocracking product stream.

One of the parameters determining the benzene yield from a hydrocrackingfeed stream comprising BTX and ethylbenzene can be expressed as:[molar amount of benzene+molar amount of ethylbenzene]*[molar amount ofxylene]/[molar amount of toluene]² (herein sometimes expressed as[BX]/T²)

All amounts in this formula are amounts in the hydrocracking feedstream.

It was found that this parameter can be used as a good indicator forpredicting the obtained benzene amount as a result of the toluenedisproportionation during hydrocracking. Since it was experimentallyfound that ethylbenzene is mostly converted to benzene by thehydrocracking step of the invention, the amount of ethylbenzene is addedto the amount of benzene in the above formula. Controlling of the[BX]/T² in the hydrocracking feed stream can easily be done by adjustingthe amount of toluene to be recycled. This ensures that a suitableamount of toluene is recycled back for obtaining a good balance betweenbenzene yield and sufficient hydrocracking activity for a reasonablysized reactor with a reasonable CAPEX.

Typically, [BX]/T² is at most 20, more typically at most 10, moretypically at most 5, more typically at most 1. Preferably, [BX]/T² is atmost 0.5, more preferably at most 0.35, more preferably at most 0.25,more preferably at most 0.15. This results in a high benzene yield inthe hydrocracking product stream. In order to ensure sufficienthydrocracking activity of the catalyst, [BX]/T² is preferably higherthan zero. Preferably, [BX]/T² is at least 0.02, for example at least0.05. The parameter [BX]/T² may generally be lower for a larger reactor.The range of [BX]/T² of 0.05 to 0.2 may be optimal for the balancebetween the good benzene yield and sufficient hydrocracking activity fora reasonably sized reactor with a reasonable CAPEX.

It is noted that U.S. Pat. No. 3,957,621 mentions recycling tolueneproduced by the hydrocracking to be added to the fresh feed. In tableVII of U.S. Pat. No. 3,957,621, the composition of the fresh feed and ofthe hydrocracking product stream (effluent after the hydrocracking) isshown. The composition of the hydrocracking feed stream, i.e. themixture of the fresh feed and the recycled toluene to be hydrocracked,is not shown. [BX]/T² in the hydrocracking feed stream, which iscritical for the balance between benzene yield and sufficienthydrocracking activity, is not mentioned in U.S. Pat. No. 3,957,621.U.S. Pat. No. 3,957,621 does not teach the idea of controlling of the[BX]/T² in the hydrocracking feed stream. Furthermore, benzene is alwaysremoved from the stream to be hydrocracked in U.S. Pat. No. 3,957,621.

Hydrocracking Catalyst

The catalyst according to the invention is a hydrocracking catalystcomprising 0.01-1 wt-% hydrogenation metal and a zeolite having a poresize of 5-8 Å and a silica (SiO₂) to alumina (Al₂O₃) molar ratio of5-200 and the process conditions comprise a temperature of 425-580° C.,a pressure of 300-5000 kPa gauge and a Weight Hourly Space Velocity of0.1-15 h⁻¹ to produce the hydrocracking product stream.

In preferred embodiments, the hydrocracking catalyst further has ahydrodesulphurisation activity. This is advantageous in that it is notnecessary to subject the feed stream to a desulphurisation treatmentprior to subjecting said hydrocarbon feed stream to the hydrocrackingtreatment.

Catalysts having hydrocracking/hydrodesulphurisation activity(“hydrocracking/hydrodesulphurisation catalyst”) are described on pages13-14 and 174 of Hydrocracking Science and Technology (1996) Ed. JuliusScherzer, A. J. Gruia, Pub. Taylor and Francis. Hydrocracking andhydrodesulphurisation reactions proceed through a bifunctional mechanismwhich requires a relatively strong acid function, which provides for thecracking and isomerization and which provides breaking of thesulphur-carbon bonds comprised in the organic sulphur compoundscomprised in the feed, and a metal function, which provides for theolefin hydrogenation and the formation of hydrogen sulphide. Manycatalysts used for the hydrocracking/hydrodesulphurisation process areformed by composting various transition metals with the solid supportsuch as alumina, silica, alumina-silica, magnesia and zeolites.

It is a particular advantage of the process of the invention that thehydrocracking product stream is substantially free from non-aromatic C6+hydrocarbons as these hydrocarbons usually have boiling points close tothe boiling point of C6+ aromatic hydrocarbons. Hence, it can bedifficult to separate the non-aromatic C6+ hydrocarbons from thearomatic C6+ hydrocarbons comprised in the hydrocracking product streamby distillation.

These advantageous effects are obtained by strategically selecting thehydrocracking catalyst in combination with the hydrocracking conditions.By combining a hydrocracking catalyst having a relatively strong acidfunction (e.g. by selecting a catalyst comprising a zeolite having apore size of 5-8 Å and a silica (SiO₂) to alumina (Al₂O₃) molar ratio of5-200) and a relatively strong hydrogenation activity (e.g. by selectinga catalyst comprising 0.01-1 wt-% hydrogenation metal) with processconditions comprising a relatively high process temperature (e.g. byselecting a temperature of 425-580° C.), chemical grade BTX can beproduced from a mixed C5-C12 hydrocarbon feed stream, wherein theconversion of the benzene comprised in the feed stream to otherhydrocarbon compounds such as naphthene compounds is reduced.

The hydrocracking of the feed stream is performed at a pressure of300-5000 kPa gauge, preferably at a pressure of 600-3000 kPa gauge, morepreferably at a pressure of 1000-2000 kPa gauge and most preferably at apressure of 1200-1600 kPa gauge. By increasing reactor pressure,conversion of C5+ non-aromatics can be increased, but also increases theyield of methane and the hydrogenation of aromatic rings to cyclohexanespecies which can be cracked to LPG species. This results in a reductionin aromatic yield as the pressure is increased, and as some cyclohexaneand its isomer methylcyclopentane, are not fully hydrocracked, there isan optimum in the purity of the resultant benzene at a pressure of1200-1600 kPa.

The hydrocracking/hydrodesulphurisation of the feed stream is performedat a Weight Hourly Space Velocity (WHSV) of 0.1-15 h⁻¹, preferably at aWeight Hourly Space Velocity of 1-10 h⁻¹ and more preferably at a WeightHourly Space Velocity of 2-9 h⁻¹. When the space velocity is too high,not all BTX co-boiling paraffin components are hydrocracked, so it willnot be possible to achieve BTX specification by simple distillation ofthe reactor product. At too low space velocity the yield of methanerises at the expense of propane and butane. By selecting the optimalWeight Hourly Space Velocity, it was surprisingly found thatsufficiently complete reaction of the benzene co-boilers is achieved toproduce on spec BTX without the need for a liquid recycle.

Accordingly, the hydrocracking conditions thus include a temperature of425-580° C., a pressure of 300-5000 kPa gauge and a Weight Hourly SpaceVelocity of 0.1-15 h⁻¹. Preferred hydrocracking conditions include atemperature of 450-550° C., a pressure of 600-3000 kPa gauge and aWeight Hourly Space Velocity of 1-10 h⁻¹. More preferred hydrocrackingconditions include a temperature of 450-550° C., a pressure of 1000-2000kPa gauge and a Weight Hourly Space Velocity of 2-9 h⁻¹.

Hydrocracking catalysts that are particularly suitable for the processof the present invention comprise a molecular sieve, preferably azeolite, having a pore size of 5-8 Å.

Zeolites are well-known molecular sieves having a well-defined poresize. As used herein, the term “zeolite” or “aluminosilicate zeolite”relates to an aluminosilicate molecular sieve. An overview of theircharacteristics is for example provided by the chapter on MolecularSieves in Kirk-Othmer Encyclopedia of Chemical Technology, Volume 16, p811-853; in Atlas of Zeolite Framework Types, 5th edition, (Elsevier,2001). Preferably, the hydrocracking catalyst comprises a medium poresize aluminosilicate zeolite or a large pore size aluminosilicatezeolite. Suitable zeolites include, but are not limited to, ZSM-5,MCM-22, ZSM-11, beta zeolite, EU-1 zeolite, zeolite Y, faujastite,ferrierite and mordenite. The term “medium pore zeolite” is commonlyused in the field of zeolite catalysts. Accordingly, a medium pore sizezeolite is a zeolite having a pore size of about 5-6 Å. Suitable mediumpore size zeolites are 10-ring zeolites, i.e. the pore is formed by aring consisting of 10 SiO₄ tetrahedra. Suitable large pore size zeoliteshave a pore size of about 6-8 Å and are of the 12-ring structure type.Zeolites of the 8-ring structure type are called small pore sizezeolites. In the above cited Atlas of Zeolite Framework Types variouszeolites are listed based on ring structure. Most preferably the zeoliteis ZSM-5 zeolite, which is a well-known zeolite having MFI structure.ZSM-5 zeolite is preferred for the high purity of BTX.

Preferably, the silica to alimuna ratio of the ZSM-5 zeolite is in therange of 20-200, more preferably in the range of 30-100.

The zeolite is in the hydrogen form: i.e. having at least a portion ofthe original cations associated therewith replaced by hydrogen. Methodsto convert an aluminosilicate zeolite to the hydrogen form are wellknown in the art. A first method involves direct ion exchange employingan acid and/or salt. A second method involves base-exchange usingammonium salts followed by calcination.

Furthermore, the catalyst composition comprises a sufficient amount ofhydrogenation metal to ensure that the catalyst has a relatively stronghydrogenation activity. Hydrogenation metals are well known in the artof petrochemical catalysts.

It is preferred that the catalyst does not comprise secondary metals,such as tin, lead or bismuth, that inhibit the hydrogenation activity ofthe hydrogenation metal. Preferably, the hydrocracking catalyst used inthe process of the present invention (the first hydrocracking catalystand the second hydrocracking catalyst) accordingly comprises less than0.01 parts tin and less than 0.02 parts lead and less than 0.01 partsbismuth (on the basis of 100 parts by weight of the total catalyst),preferably less than 0.005 parts tin and less than 0.01 parts lead andless than 0.005 parts bismuth (on the basis of 100 parts by weight oftotal catalyst).

The catalyst composition comprises 0.01-1 wt-% hydrogenation metal,preferably 0.01-0.7 wt-%, more preferably 0.01-0.5 wt-% hydrogenationmetal, more preferably 0.01-0.3 wt-%. The catalyst composition maycomprise 0.01-0.1 wt-% or 0.02-0.09 wt-% hydrogenation metal. In thecontext of the present invention, the term “wt %” when relating to themetal content as comprised in a catalyst composition relates to the wt %(or “wt-%”) of said metal in relation to the weight of the totalcatalyst, including catalyst binders, fillers, diluents and the like.Preferably, the hydrogenation metal is at least one element selectedfrom Group 10 of the periodic table of Elements. The preferred Group 10element is platinum.

Accordingly, the hydrocracking catalyst used in the process of thepresent invention preferably comprises a zeolite having a pore size of5-8 Å, a silica (SiO₂) to alumina (Al₂O₃) molar ratio of 5-200 and0.01-1 wt-% platinum (in relation to the total catalyst).

The hydrocracking catalyst composition may further comprise a binder.Alumina (Al₂O₃) is a preferred binder. The catalyst composition of thepresent invention preferably comprises at least 10 wt-%, most preferablyat least 20 wt-% binder and more preferably comprises up to 40 wt-%binder in relation to the total amount of the catalyst. In oneembodiment, the hydrogenation metal is deposited on the binder, whichpreferably is Al₂O₃. In some embodiments, the catalyst composition ofthe present invention comprises little or no binder, for example lessthan 2 wt %, less than 1 wt %, less than 0.5 wt % or 0 wt %.

According to one embodiment of the invention the hydrocracking catalystis a mixture of the hydrogenation metal on a support of an amorphousalumina and the zeolite. In this case, the hydrocracking catalyst can bemade by physically mixing the zeolite with the amorphous alumina onwhich the hydrogenation metal is present.

According to another embodiment of the invention the hydrocrackingcatalyst comprises the hydrogenation metal on a zeolite-based support.In this case, the hydrogenation metal is deposited on the zeolite whichacts as a support. The deposition of a hydrogenation metal on a zeoliteis well-known and can e.g. be done by wet impregnation, base exchange orion exchange. The presence of the hydrogenation metal on the zeolite maye.g. be determined by Energy Dispersive X-Ray Spectroscopy (EDS)(through use of SEM), CO chemisorption, inductively coupled plasmaatomic emission spectroscopy (ICP-AES) or X-Ray Fluorescence (XRF). Inthis case, the hydrogenation metal and the zeolite, which gives crackingfunctionality, are in closer proximity to one another, which translatesinto a shorter diffusion length between the two sites. This allows highspace velocity, which translates into smaller reactor volumes and thuslower CAPEX. Accordingly, in some preferred embodiments, thehydrocracking catalyst is the hydrogenation metal on a support of thezeolite and step (b) is performed at a Weight Hourly Space Velocity of1-15 or 10-15 h⁻¹.

The hydrocracking step is performed in the presence of an excess amountof hydrogen in the reaction mixture. This means that a more thanstoichiometric amount of hydrogen is present in the reaction mixturethat is subjected to hydrocracking. Preferably, the molar ratio ofhydrogen to hydrocarbon species (H₂/HC molar ratio) in the reactor feedis between 1:1 and 4:1, preferably between 1:1 and 3:1 and mostpreferably between 1:1 and 2:1. A higher benzene purity in the productstream can be obtained by selecting a relatively low H₂/HC molar ratio.In this context the term “hydrocarbon species” means all hydrocarbonmolecules present in the reactor feed such as benzene, toluene, hexane,cyclohexane etc. It is necessary to know the composition of the feed tothen calculate the average molecular weight of this stream to be able tocalculate the correct hydrogen feed rate. The excess amount of hydrogenin the reaction mixture suppresses the coke formation which is believedto lead to catalyst deactivation.

Step c)

According to step c) of the process according to the invention, benzene,toluene and C8+ hydrocarbons are separated from the hydrocrackingproduct stream.

The hydrocracking product stream comprises methane, LPG, benzene,toluene and C8+ hydrocarbons. The term “LPG” as used herein refers tothe well-established acronym for the term “liquefied petroleum gas”. LPGgenerally consists of a blend of C2-C4 hydrocarbons i.e. a mixture ofC2, C3, and C4 hydrocarbons. The hydrocracking product stream may besubjected to separation by standard means and methods suitable forseparating methane and unreacted hydrogen comprised in the hydrocrackingproduct stream as a first separate stream, the LPG comprised in thehydrocracking product stream as a second separate stream and benzene,toluene and C8+ hydrocarbons as a third separate stream. Preferably, thestream comprising benzene, toluene and C8+ hydrocarbons is separatedfrom the hydrocracking product stream by gas-liquid separation ordistillation. One non-limiting example of such a separation methodincludes a series of distillation steps. The first distillation step atmoderate temperature is to separate most of the aromatic species (liquidproduct) from the hydrogen, H₂S, methane and LPG species. The gaseousstream from this distillation is further cooled (to about −30° C.) anddistilled again to separate the remaining aromatics species and most ofthe propane and butane. The gaseous product (mainly hydrogen, H₂S,methane and ethane) is then further cooled (to about −100° C.) toseparate the ethane and leave the hydrogen, H₂S and methane in thegaseous stream that will be recycled back to the hydrocracking reactor.To control the levels of H₂S and methane in the reactor feed, aproportion of recycle gas stream is removed from the system as a purge.The quantity of material that is purged depends on the levels of methaneand H₂S in the recycle stream which in-turn depend on the feedcomposition. The purge stream will have the same composition as therecycle stream. As the purge will contain mainly hydrogen and methane itis suitable for use as a fuel gas or may be further treated (e.g. via apressure swing adsorption unit) to separately recover a high purityhydrogen stream and a methane/H₂S stream which can be used as a fuelgas.

Step d)

According to step d) of the process according to the invention, benzeneand toluene are separated from the stream of benzene, toluene and C8+hydrocarbons.

Benzene and toluene are separated from each other by gas-liquidseparation or distillation. Chemical grade benzene and chemical gradetoluene are obtained.

At least part of the obtained toluene is recycled back to be included inthe hydrocracking feed stream. The amount of the toluene that is to beincluded in the feed hydrocracking stream can be varied in order tooptimize the amount of benzene in the hydrocracking product stream.

Xylene may be separated from the C8+ hydrocarbon. Preferably, theseparation is done by gas-liquid separation or distillation.

Although the invention has been described in detail for purposes ofillustration, it is understood that such detail is solely for thatpurpose and variations can be made therein by those skilled in the artwithout departing from the spirit and scope of the invention as definedin the claims.

It is further noted that the invention relates to all possiblecombinations of features described herein, preferred in particular arethose combinations of features that are present in the claims.

It is noted that the term “comprising” does not exclude the presence ofother elements. However, it is also to be understood that a descriptionon a product comprising certain components also discloses a productconsisting of these components. Similarly, it is also to be understoodthat a description on a process comprising certain steps also disclosesa process consisting of these steps.

The present invention will now be elucidated by the followingnon-limiting Examples.

The FIGURE shows a scheme illustrating an example of the processaccording to the invention. In this example, the source feed stream(pyrolysis gasoline) is contacted with hydrogen and treated in ahydrogenation reactor. After the hydrogenation, additional hydrogen issupplied and the hydrogenated stream is combined with the toluene thatis recycled back and the hydrocracking feed stream thus obtained isintroduced into the hydrocracking reactor (MHC). The hydrocrackingproduct stream is subjected to gas-liquid separation to be separatedinto a gas phase of hydrogen, methane and LPG and a liquid phase ofbenzene, toluene and C8+ hydrocarbons. The liquid phase is thereafterseparated into benzene, toluene and C8+ hydrocarbons by simpledistillation. Part of the toluene obtained is recycled back to becombined with the hydrogenated stream.

EXAMPLES

Feed mixtures comprising different compositions of hydrocarbons weresubjected to hydrocracking in order to determine the influence of thefeed compositions to the product compositions. The experiments werecarried out in a 12 mm reactor, wherein the catalyst bed was located inthe isothermal zone of the reactor heater. The catalyst used was 2.0 gof Pt deposited on ZSM-5, wherein SiO₂/Al₂O₃=50. The amount of Pt was0.08 wt % with respect to the total of Pt and ZSM-5. No binder was usedin the catalyst.

The feed streams were fed to the reactor. The feed stream enters avaporizer section prior to the reactor where it is vaporized at 280° C.and mixed with hydrogen gas. The conditions used throughout theseexperiments were: WHSV=6/hr, pressure was 1379 kPa (200 psig),temperature was 475° C. and the molar ratio H₂/hydrocarbons was 3. Theeffluent of the reactor was sampled in the gas phase to an online gaschromatograph. Product analyses were carried out once per hour.

In experiment 1, different feed streams (Feed 1-5) obtained by addingtoluene to pyrolysis gasoline samples were subjected to hydrocracking.The compositions of the feed streams are given in Table 1. Thehydrocracking resulted in hydrocracking product streams (Product 1-5)with compositions according to Table 2. By comparing the results inTable 2 it was observed that when the wt % of toluene in the feed streamrises and the [BX]/T² molar value (the ratio of [the product of themolar amounts of (benzene+ethylbenzene) and xylene] to [the molar amountof toluene]²) is reduced, the overall benzene yield is increased duringhydrocracking. Benzene loss was almost zero at [BX]/T² of 0.16. Abenzene gain was obtained at [BX]/T² of 0.09.

TABLE 1 Compositions of hydrocracking feed stream Component Feed #1 Feed#2 Feed #3 Feed #4 Feed #5 Benzene (wt %) 51.64% 50.14% 46.75% 43.28%39.86% Toluene (wt %) 12.73% 15.64% 21.39% 27.35% 33.13% Ethylbenzene(wt %) 4.19% 4.04% 3.79% 3.51% 3.21% BTX (wt %) 67.35% 68.64% 70.80%73.10% 75.25% Total Aromatics (wt %) 72.11% 73.20% 75.08% 77.06% 78.86%[BX]/T² (molar) 1.03 0.64 0.30 0.16 0.09

TABLE 2 Compositions of hydrocracking product stream Component Product#1 Product #2 Product #3 Product #4 Product #5 Benzene (wt %) 48.29%47.25% 45.28% 43.17% 41.28% Toluene (wt %) 18.78% 20.42% 23.47% 26.56%29.35% Ethylbenzene (wt %) 0.34% 0.35% 0.32% 0.30% 0.26% BTX (wt %)69.89% 70.86% 72.62% 74.50% 76.23% Total Aromatics (wt %) 70.84% 71.83%73.64% 75.46% 77.17% [BX]/T² (molar) 0.41 0.39 0.34 0.31 0.29 BenzenePurity (%) 99.90% 99.89% 99.88% 99.88% 99.88% Toluene Gain/Loss (%)+47.52% +30.56% +9.72% −2.89% −11.41% Toluene Gain/Loss (moles) +0.066+0.052 +0.023 −0.009 −0.041 Benzene Gain/Loss (%) −6.49% −5.76% −3.14%−0.25% +3.56% Benzene Gain/Loss (moles) −0.043 −0.037 −0.019 −0.001+0.018

Benzene purity is defined as [mass of benzene]/[sum of the masses ofbenzene, 2-methylpentane, 3-methylpentane, hexane, methylcyclopentaneand cyclohexane].

The invention claimed is:
 1. A process for producing benzene, comprisingthe steps of: (a) providing a hydrocracking feed stream comprisingC5-C12 hydrocarbons, (b) contacting the hydrocracking feed stream in thepresence of hydrogen with a hydrocracking catalyst to produce ahydrocracking product stream comprising benzene, toluene and C8+hydrocarbons, (c) separating benzene, toluene and the C8+ hydrocarbonsfrom the hydrocracking product stream, and (d) selectively recyclingpart of the toluene from the separated products of step (c) to thehydrocracking feed stream, wherein the hydrocracking feed stream has aweight ratio of benzene to toluene of less than 1.58:1.
 2. The processaccording to claim 1, wherein the hydrocracking feed stream has a weightratio of benzene to toluene of less than or equal to 1.20:1.
 3. Theprocess according to claim 1, wherein an amount of the toluene recycledto the hydrocracking feed stream is 10-80 wt % of the toluene in thehydrocracking product stream.
 4. The process according to claim 1,wherein the amount of the toluene recycled to the hydrocracking feedstream is 50-70 wt % of the toluene in the hydrocracking product stream.5. The process according to claim 1, wherein step (a) involves mixing afresh feed stream and the recycled toluene, wherein the fresh feedstream comprises at least 10 wt % benzene.
 6. The process according toclaim 5, wherein the fresh feed stream is pyrolysis gasoline, straightrun naphtha, light coker naphtha, coke oven light oil or mixturesthereof, optionally after being hydrogenated in a hydrogenation reactor.7. The process according to claim 5, wherein the fresh feed streamcomprises at least 20 wt % benzene.
 8. The process according to claim 7,wherein the fresh feed stream comprises at least 30 wt % benzene.
 9. Theprocess according to claim 1, wherein the hydrocracking feed stream hasnot been subjected to the step of removing benzene.
 10. The processaccording to claim 1, wherein the hydrocracking feed stream comprises atleast 1 wt % and at most 20 wt % benzene.
 11. The process according toclaim 1, wherein the amount of toluene to be recycled to thehydrocracking feed stream is set so that the proportion of benzene inthe hydrocracking product stream is 0-5 mol % higher than the proportionof benzene in the hydrocracking feed stream.
 12. The process accordingto claim 1, wherein xylene is separated from the hydrocracking productstream.
 13. The process according to claim 1, wherein the amount oftoluene to be recycled to the hydrocracking feed stream is set so thatthe ratio of [molar amount of benzene+molar amount ofethylbenzene]*[molar amount of xylene] divided by [molar amount oftoluene]² in the hydrocracking feed stream is 0.02 to
 20. 14. Theprocess according to claim 1, wherein the amount of toluene to berecycled to the hydrocracking feed stream is set so that the ratio of[molar amount of benzene+molar amount of ethylbenzene]*[molar amount ofxylene] divided by [molar amount of toluene]² in the hydrocracking feedstream is 0.05 to 0.15.
 15. The process according to claim 1, whereincontacting the hydrocracking feed stream in the presence of hydrogenwith the hydrocracking catalyst comprises process conditions including atemperature of 425-580° C., a pressure of 300-5000 kPa gauge and aWeight Hourly Space Velocity of 0.1-15 h⁻¹.
 16. The process according toclaim 15, wherein the hydrocracking catalyst comprises 0.01-1 wt-%hydrogenation metal in relation to the total catalyst weight and azeolite having a pore size of 5-8 Å and a silica (SiO₂) to alumina(Al₂O₃) molar ratio of 5-200.
 17. The process according to claim 16,wherein the hydrocracking catalyst comprises less than 0.01 parts tin,less than 0.02 parts lead, and less than 0.01 parts bismuth on the basisof 100 parts by weight of the total catalyst.
 18. The process accordingto claim 16, wherein the zeolite is a ZSM-5 zeolite.
 19. The processaccording to claim 16, wherein the hydrogenation metal is platinum. 20.The process according to claim 16, wherein the hydrocracking catalystcomprises the hydrogenation metal on a zeolite-based support.